Conditions that perturb the function of the endoplasmic reticulum (ER) lead to an accumulation of proteins and subsequent induction of several responses, such as an increased expression of ER-resident chaperones involved in protein folding and activation of c-jun N-terminal kinase (JNK). These responses are mediated by a transmembrane kinase/ribonuclease, IRE1, which transduces the signal from the ER lumen to the cytosol. Although nuclear transcription factor-kappaB (NF-kappaB) is also activated by ER stress, whether this response depends on IRE1 is unknown. In this study, we show that IRE1 is involved in the activation of NF-kappaB induced by ER stress. NF-kappaB was activated by ER stress-inducing agents, thapsigargin and tunicamycin. The activation was inhibited by a dominant-negative IRE1. In addition, a dominant-negative TRAF2 also suppressed the activation of NF-kappaB by ER stress. These results suggest that ER stress-induced NF-kappaB activation is also mediated by the IRE1-TRAF2 pathway, as well as JNK activation.

Sodium 4-phenylbutyrate (4-PBA) is a low molecular weight fatty acid that has been used for treatment of urea cycle disorders in children, sickle cell disease, and thalassemia. It has been demonstrated recently that 4-PBA can act as a chemical chaperone by reducing the load of mutant or mislocated proteins retained in the endoplasmic reticulum (ER) under conditions associated with cystic fibrosis and liver injury. In the present study, we evaluated the neuroprotective effect of 4-PBA on cerebral ischemic injury. Pre- or post-treatment with 4-PBA at therapeutic doses attenuated infarction volume, hemispheric swelling, and apoptosis and improved neurological status in a mouse model of hypoxia-ischemia. Moreover, 4-PBA suppressed ER-mediated apoptosis by inhibiting eukaryotic initiation factor 2α phosphorylation, CCAAT/enhancer-binding protein homologous protein induction, and caspase-12 activation. In neuroblastoma neuro2a cells, 4-PBA reduced caspase-12 activation, DNA fragmentation, and cell death induced by hypoxia/reoxygenation. It protected against ER stress-induced but not mitochondria-mediated cell death. Additionally, 4-PBA inhibited the expression of inducible nitric-oxide synthase and tumor necrosis factor-α in primary cultured glial cells under hypoxia/reoxygenation. These results indicate that 4-PBA could protect against cerebral ischemia through inhibition of ER stress-mediated apoptosis and inflammation. Therefore, the multiple actions of 4-PBA may provide a strong effect in treatment of cerebral ischemia, and its use as a chemical chaperone would provide a novel approach for the treatment of stroke.

Endoplasmic reticulum-associated degradation (ERAD) is a system by which proteins accumulated in the endoplasmic reticulum (ER) are retrotranslocated to the cytosol and degraded by the ubiquitin–proteasome pathway. HRD1 is expressed in brain neurons and acts as an ERAD ubiquitin ligase. Amyloid precursor protein (APP) is processed into amyloid-β peptides (Aβs) that form plaque deposits in the brains of Alzheimer's disease (AD) patients. We found significantly decreased HRD1 protein levels in the cerebral cortex of AD patients. HRD1 colocalized with APP in brain neurons and interacted with APP through the proline-rich region of HRD1. HRD1 promoted APP ubiquitination and degradation, resulting in decreased generation of Aβ. Furthermore, suppression of HRD1 expression induced APP accumulation that led to increased production of Aβ associated with ER stress. Immunohistochemical analysis revealed that suppression of HRD1 expression inhibited APP aggresome formation, resulting in apoptosis. In addition, we found that the ATF6- and XBP1-induced upregulation of ERAD led to APP degradation and reduced Aβ production. These results suggest that the breakdown of HRD1-mediated ERAD causes Aβ generation and ER stress, possibly linked to AD.

Endoplasmic reticulum (ER) stress is defined as an accumulation of unfolded proteins in the endoplasmic reticulum. 4‐phenylbutyrate (4‐PBA) has been demonstrated to promote the normal trafficking of the ΔF508 cystic fibrosis transmembrane conductance regulator (CFTR) mutant from the ER to the plasma membrane and to restore activity. We have reported that 4‐PBA protected against cerebral ischemic injury and ER stress‐induced neuronal cell death. In this study, we revealed that 4‐PBA possesses chemical chaperone activityin vitro, which prevents the aggregation of denatured α‐lactalbumin and bovine serum albumin (BSA). Furthermore, we investigated the effects of 4‐PBA on the accumulation of Parkin‐associated endothelin receptor‐like receptor (Pael‐R) pathologically relevant to the loss of dopaminergic neurons in autosomal recessive juvenile parkinsonism (AR‐JP). Interestingly, 4‐PBA restored the normal expression of Pael‐R protein and suppressed ER stress induced by the overexpression of Pael‐R. In addition, we showed that 4‐PBA attenuated the activation of ER stress‐induced signal transduction pathways and subsequent neuronal cell death. Moreover, 4‐PBA restored the viability of yeasts that fail to induce an ER stress response under ER stress conditions. These results suggest that 4‐PBA suppresses ER stress by directly reducing the amount of misfolded protein, including Pael‐R accumulated in the ER.